Help identifying Pacific Northwest snake

[portcallahan]

My dad found this 5' snake near his driveway. It was dead, run over by a car. He lives in the Tacoma/Puget Sound region of Washington about a mile away from a freshwater lake. He has two acres that are surrounded by lots of large fir trees. Unfortunately, he didn't get a photo of the head. It was a full 5' long and very scaly.

Anyone have ideas what this could be? We don't know if it came from the yard or fell off of a car or boat trailer. I just want to make sure it's safe for my kids to play in the wooded areas of my Dad's yard.

Thanks much!